Plants for heat treating granular raw material such as cement raw meal often include one or more vertical multi-stage suspension preheater strings each of which includes a plurality of serially connected cyclone separators that receive cement raw material at the top and hot exhaust gases from the rotary kiln at the bottom with countercurrent flow of the hot gases and the cement raw meal through the preheater to thereby preheat the raw cement feed for the rotary kiln. A typical calcining cement suspension preheater string such as disclosed, for example, in U.S. Pat. Nos. 3,891,383; 3,904,353 and 3,914,098 may include four serially connected cyclone separators interconnected by heat exchanger conduits and meal pipes to achieve four stages of heat exchange. Each cyclone separator has an inlet for hot gas and suspended raw meal, an outlet at its upper end for separated hot gases connected to a stage above, and an outlet at its lower end for separated raw cement meal connected to a stage below. The cyclone separators in the suspension preheater string are spaced apart vertically, and the raw cement meal flows by gravity through meal pipes interconnecting the cyclone separators while kiln off gas is moved upwardly through the separators and heat exchange conduits by suction from an induced draft fan.
Cement plants of such multi-stage preheater strings are of extreme vertical height, for example, 190 feet height for a four stage cyclone-type suspension preheater, and the vertical dimension of each cyclone separator contributes significantly to the undesirable height of a conventional cyclone-type suspension preheater tower.
A conventional cyclone separator has a relatively high gas pressure drop and relatively high friction loss which result in high energy losses in the calcining cement suspension preheater and necessitate use of high horsepower induced draft fans. Power consumption in a cyclone type preheater results principally from moving the gas against the pressure differential of the cyclone separators. A major portion of the gas pressure drop in a cyclone separator results from: (a) the energy required to draw the relatively low whirl velocity gas at the cyclone body diameter into the higher whirl velocity of the gas exit pipe diameter, and (b) the unrecovered energy of the higher whirl velocity of the exit gas stream. Cyclone separator type suspension preheater strings are usually limited to four stages because of height and pressure drop limitations.
U.S. Pat. No. 3,049,343 to Helming discloses a cement plant preheater wherein the axes of the cyclone separators are inclined at a 45 degree angle to the horizontal for the purpose of reducing the height of the preheater tower, but such arrangement has not proven commercially successful and has the above discussed disadvantage of cyclone separators.
U.S. Pat. No. 3,358,426 to Husbjerg discloses apparatus for preheating cement raw meal and separating the hot meal particles from the gas after heat transfer which utilizes centrifugal force to throw the meal particles against the outer wall of a curved pipe and precipitate them from the gas, but the apparatus disclosed in this prior art patent has only a single stage of heat transfer and would be capable of carrying out only a relatively small percent of the calcination of the cement meal.